(1) Field of the Invention
The present invention generally relates to radio frequency (RF) power amplifier output power detection techniques and, more particularly, to a system and method for sensing power consumed by a RF power amplifier and compensating for bias changes.
(2) Description of the Prior Art
Portable wireless transmitter systems generally require that a specified radio frequency (RF) output power be delivered to the radiating antenna. Further, many such systems are required to back-off or reduce the transmitted power to achieve a specific level depending upon signal strength. To meet the aforesaid requirements, the system architecture generally incorporates a closed-loop power control scheme. Typically, this scheme requires a "sampling" of the RF power amplifier output power that is subsequently fed back to predetermined control circuitry which generates a control signal that adjusts the output power until it is within the specified power level. Such sampling of the output power is disadvantageous in that it increases the insertion loss between the output of the power amplifier and the radiating antenna. Therefore, sampling of the output power decreases the available output power from the power amplifier and reduces the overall talk time. Talk time is a measure of the time a portable transceiver can be in the "talk" mode before the battery is fully depleted. The power amplifier consumes the majority of the current and therefore dominates in the calculation of talk time.
A common technique for sampling the output power includes the use of a directional coupler on the output of the power amplifier. The power coupled from the main signal path is diode detected to generate a video signal proportional to the amplitude of the RF voltage delivered to the antenna. Use of directional couplers, however, adds loss to the system, forcing the power amplifier to consume more power thereby reducing the talk time of the associated radio unit. In typical applications, the aforesaid loss is often 5-10% of the power amplifier output power and relates to a direct loss in available talk time.
Another common technique for detecting the output power includes measurement of the current consumed by the power amplifier. This current is directly related to the output power generated by the power amplifier and is also fed back to predetermined power control leveling circuitry. This technique is also disadvantageous due to the loss associated with the current measurement. This current measurement generally requires that a series "dropping" element be added between the associated battery and the power amplifier bias input. The voltage across this element will determine the current entering the power amplifier (for a known resistance across the element). In typical applications, the voltage across the dropping element will be about 3% of the total battery voltage. Because this is a loss in the de input power to the power amplifier, the loss of talk time will be even higher than 3% due to the less than 100% dc-rf conversion efficiency of the power amplifier. For example, if the power amplifier efficiency is 60%, then the talk time loss will be 3/0.6+L or 5%.
Thus, there remains a need for a new and improved technique for current sensing associated with RF amplifier power detection.
SUMMARY OF THE INVENTION
The present invention is directed to a system and method relating to current sensing to detect RF amplifier output power and controlling a bias circuit to ensure that a bias current is set to a desired value. A small current sensing transistor is added in parallel with a large transistor that delivers the high power to the antenna. The current in this sensing transistor is proportional to the current in the large transistor. The current in the large transistor is directly related to the output power generated by the power amplifier. By using a small current sensing transistor, talk time loss is reduced by less than 1%. This small transistor is dc biased and RF driven proportionally to the large output transistor. The small transistor (sampling transistor) is about 1/250+L .sub.th the size of the larger output transistor which results in a scaling factor such that the additional current required for sensing is about 0.4% of the total current consumed by the power amplifier.
In order to compensate for a dominating quiescent bias current at low power levels, an embodiment of the present invention incorporates a second small current sensing transistor that is in parallel with the large transistor and the first small current sensing transistor. However, the second small current sensing transistor does not receive the RF signal driving the large transistor and the first small current sensing transistor. Rather, the second small current sensing transistor only receives a bias current. Preferably, the second small transistor is the same physical size as the first small transistor. This configuration allows the power sensed by the first small current sensing transistor to be corrected for bias current effects and bias current shifts.
In a further embodiment of the present invention, an operational amplifier is used in a feedback loop to precisely set the amplifier bias point for the large transistor. More specifically, the operational amplifier is input with a reference voltage proportional to a desired bias current in the large transistor and the detected de bias of the second small transistor. The output of the operational amplifier is fed back to the large transistor.
Accordingly, one feature of the present invention includes a technique for measurement of RF amplifier output power that reduces talk time by less than 1%.
Another feature of the present invention includes a technique for measurement of RF amplifier output power that is more efficient than known measurement techniques.
Still another feature of the present invention includes a technique for measurement of RF amplifier output power which consumes about an order of magnitude less power than that consumed by using known techniques.
Yet another feature of the present invention includes a technique for measurement of RF, amplifier output power that compensates for bias current effects and bias current shifts.
A further feature of the present invention includes a technique for controlling the total bias current to an RF amplifier transistor by sensing the output power and the bias.
These and other features of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiment when considered with the drawings.